(First Prior Art)
First prior art is described. Friction on a sliding part employed for a Stirling engine remarkably influences performance and reliability of the Stirling engine, and hence a conventional Stirling engine employs a gas effusion structure utilizing a gas bearing effect for a sliding part thereby reducing friction on the sliding part.
The following two examples can be generally listed as the conventional gas effusion structure utilizing a gas bearing effect. FIG. 31 schematically shows a first gas effusion structure. As shown in this figure, a small hole 121 formed by drilling is provided on a gas outlet of a piston 103 which is a motional body provided in a cylinder 102 for effusing gas from this small hole 121 thereby forming a hydrostatic gas bearing between sliding surfaces of the cylinder 102 and the piston 103. This system is referred to as an orifice system.
FIG. 32 schematically shows a second gas effusion structure. As shown in this figure, an air-permeable porous body 122 having innumerable pores in its material is arranged on a gas outlet of a piston 103 which is an effector provided in a cylinder 102 for effusing gas from this porous body 122 thereby forming a hydrostatic gas bearing between sliding surfaces of the cylinder 102 and the piston 103.
Problems in the case of employing the aforementioned hydrostatic gas bearings employing gas bearing effects for the sliding parts of the aforementioned Stirling engines are now described.
In the orifice system according to the firs gas effusion structure shown in FIG. 31, flow loss of the gas from the gas outlet must be reduced in order to improve performance of the Stirling engine. Therefore, the pore size of the gas outlet has been remarkably reduced. However, there has been such a problem that dust in assembling of the Stirling engine or abrasive powder resulting from friction during operation flocculates and clogs the gas outlet to unidirectionally press the piston due to heterogeneity of the gas outflow from each gas outlet, leading to reduction of reliability of operation of the Stirling engine.
In the second gas effusion structure shown in FIG. 32, a large number of pores are present in the porous body 122 dissimilarly to the orifice system and hence the pore diameter of the porous body 122 must be remarkably reduced in order to narrow down the gas outflow from each gas outlet while there has been such a problem that abrasive powder or the like clogs the pores when the pore diameter is reduced.
(Second Prior Art)
Second prior art is described. FIG. 33 is a sectional view showing the structure of a conventional Stirling engine. Referring to FIG. 33, numeral 281 denotes a cylinder-like pressure vessel, and this pressure vessel 281 is filled up with high-pressure helium gas (hereinafter referred to as gas) as a medium. A columnar piston 282 having a through hole 282a is arranged in the pressure vessel 281 while matching the central axis with the pressure vessel 281, while a columnar displacer 283 having a through part 283a passing through the through hole 282a of the piston 282 on an end thereof is also arranged.
The piston 282 is linearly driven by a piston driver (not shown) consisting of a linear motor or the like in the axial direction of the pressure vessel 281, for compressing and expanding the gas in the pressure vessel 281. The piston 282 is supported by a spring 284 on an end (right end in the figure) of the pressure vessel 281 opposite to the displacer 283, not to deviate from a prescribed region.
The through part 283a is supported by a spring 285 on the end (right end in the figure) of the pressure vessel 281 so that the displacer 283 does not deviate from the prescribed region either. The piston 282 moves in the direction of the displacer 283 (leftward in the figure) thereby compressing the gas between the piston 282 and the displacer 283, so that the displacer 283 moves in the direction opposite to the piston 282 (leftward in the figure). Then, the piston 282 moves in the direction opposite to the displacer 283 (rightward in the figure) thereby expanding the gas between the piston 282 and the displacer 283, so that the displacer 283 moves in the direction of the piston 282 (rightward in the figure). The piston 282 repeats reciprocation so that the displacer 283 also repeats the aforementioned motion, for compressing and expanding the gas.
An end (left end in the figure) of the pressure vessel 281 opposite to the piston 282 side of the displacer 283 is formed as a cooling part 290, and the said cooling part 290 absorbs external heat for reducing the external temperature when the gas between the cooling part 290 and the displacer 283 is expanded.
The piston 282 and the displacer 283 reciprocate at a high speed during operation of the Stirling engine, and hence friction on sliding parts between the respective ones of the piston 282 and the displacer 283 and the pressure vessel 281 remarkably influences performance and reliability of the Stirling engine. Therefore, reduction of friction on the said sliding parts is attempted.
The structure of the piston 282 for reducing friction on the said sliding parts is now described. The displacer 283 also employs a similar structure.
The piston 282 is in the form of a column having the through hole 282a, and includes a cylindrical pressurization chamber 286 matching its central axis with the through hole 282a inside the peripheral wall. A side wall (left side in the figure) on the displacer 283 side of the piston 282 has a one-way valve 287 inwardly directed from outside the pressurization chamber 286, so that the high-pressure gas compressed by reciprocation of the piston 282 and the displacer 283 flows into and is stored in the pressurization chamber 286 through the said one-way valve 287, thereby maintaining a high pressure in the pressurization chamber 286.
A plurality (e.g., four equal-scale magnifications) of gas ports 288 are provided on a substantially central portion of the outer peripheral wall of the piston 282, and an annular porous body 289 is arranged in the pressurization chamber 286 thereby blocking open ends of the gas ports 288 closer to the pressurization chamber 286. The porous body 289 is so annularly formed as to solely block all gas ports 288.
The high-pressure gas in the pressurization chamber 286 is injected to the sliding part between the piston 282 and the pressure vessel 281 through the porous body 289 from the gas ports 288. The high-pressure gas is so injected through the porous body 289 that the porous body 289 traps dust etc. contained in the flow of the high-pressure gas while friction on the sliding part between the piston 282 and the pressure vessel 281 can be reduced by reducing the quantity of the injected gas.
The aforementioned structure is so provided in the displacer 283 that friction on the sliding part between the displacer 283 and the pressure vessel 281 can be reduced.
In the Stirling engine having the aforementioned structure, the quantities of the gas injected from the respective gas ports 288 are so uniformized that the piston 282 and the displacer 283 can stably reciprocate with low friction with respect to the pressure vessel 281.
However, adhesion between the porous body 289 and the piston 282 or the displacer 283 is not uniformized due to dispersion in shape accuracy of the piston 282, the displacer 283 and the porous body 289. Further, the piston 282 and the displacer 283 reciprocate at a high speed during operation of the Stirling engine, and hence the porous body 290 may move from a prescribed position when the said adhesion is weak. Therefore, the flow path of the gas is instable, and hence the quantities of the gas injected from the respective gas ports are so non-constant that the piston 282 and the displacer 283 cannot stably reciprocate.
(Third Prior Art)
Third prior art is described. A Stirling engine compresses and expands working gas filling up a cylinder thereby implementing a known Stirling cycle. In a crank type Stirling engine, a piston and a displacer are fixed by a shaft so that the piston and the displacer mechanically move while keeping constant relation thereby implementing the Stirling cycle. In a free-piston Stirling engine, on the other hand, a piston and a displacer are connected to/supported on a casing or the like respectively by coil springs or the like, for example, to operate with individual reciprocation characteristics. FIG. 34 shows an example of this free-piston Stirling engine.
As shown in FIG. 34, a piston 303 and a displacer 302 are coaxially engaged in a cylinder 301 having a cylindrical space therein in the free-piston Stirling engine, thereby sectionally forming a compression space 304 between the piston 303 and the displacer 302, an expansion space 305 between the displacer 302 and a closed end of the cylinder 301 and a back pressure space 306 in a space of the piston 303 opposite to the compression space 304 respectively. The compression space 304 and the expansion space 305 communicate with each other through a regenerator 307, so that working gas filling up this closed circuit serves as a working medium for a Stirling cycle.
The back pressure space 306 is also filled up with gas. However, the gas in this back pressure space 306 acts on none of a compression cycle, an expansion cycle and an isochoric cycle in the Stirling engine. In the Stirling engine, however, the amplitude center position of the piston 303 must be prevented from fluctuation and hence a communication path is generally provided for keeping pressure balance between the compression space 304 and the back pressure space 306.
For example, Japanese Patent Laying-Open No. 2000-39222 proposes a structure forming a communication path 315 by an in-piston communication path 315a provided in the piston and a communication hole 315b formed on a cylinder wall surface for coupling the in-piston communication path 316a and the communication hole 315b with each other when the piston 303 is located on its amplitude center position thereby keeping pressure balance between the compression space 304 and the back pressure space 306, as shown in FIG. 34.
When excessive gas circulates through this communication path 315, however, compressibility of the compression space 304 is reduced to cause miscellaneous loss in the Stirling engine, leading to reduction in capability. In the Stirling engine, therefore, miscellaneous loss of the Stirling engine resulting from excess gas flow must be suppressed as low as possible by controlling the flow rate of the gas circulating through the communication path 315.
In the aforementioned free-piston Stirling engine, the diameter of the communication path has been designed in response to the specifications of the Stirling engine. However, the optimum gas flow rate varies from moment to moment with the operational situation of the Stirling engine, and hence miscellaneous loss is not yet completely eliminated. If specification change is made, design of the piston itself must be restarted, leading to an enormous cost for the specification change.
In the crank type Stirling engine, a valve controlling the flow rate of the gas circulating through the communication path can be provided in the communication path due to its structure, while it is impossible to provide such a valve in the communication path in the free-piston Stirling engine.
As another factor for miscellaneous loss of the free-piston Stirling engine having the piston and displacer coaxially engaged for reciprocation, vibration of the Stirling engine itself can be listed. While a dynamic vibration damping mechanism consisting of a mass part and an elastic part can suppress this vibration of the Stirling engine itself, this results in motion loss caused by air resistance, and further results in noise. In general, absolutely no example has reduced motion loss by improving the structure of this dynamic vibration damping mechanism.
(Fourth Prior Art)
Fourth prior art is described. FIG. 35 shows the structure of a free-piston Stirling refrigerator utilizing resonance of a spring as an exemplary conventional Stirling refrigerator. A casing 414 roughly includes a working space 412 and a driving space 413. The working space 412 further consists of an expansion space 406 and a compression space 407, and the working space 412 is filled up with working gas. A first cylinder 403 is arranged along a direction connecting the expansion space 406 and the compression space 407 in the casing 414. A displacer 402 is arranged inside the first cylinder 403 to be reciprocative along the longitudinal direction of the first cylinder 403. A rod 409 extends from the displacer 402 oppositely to the expansion space 406 along the reciprocatory direction, and is elastically connected to the casing 414 by a displacer plate spring 411.
A piston 401 is arranged on a side of the displacer 402 closer to the compression space 407 to enclose the rod 409, and a second cylinder 415 is arranged to enclose the piston 401. The piston 401 is driven by a linear motor 408 arranged in the driving space 413, to be reciprocative for expanding and compressing the compression space 407 in the second cylinder 415 in a prescribed cycle. The piston 401 is elastically connected to the casing 414 by a piston plate spring 410. The displacer 402 is so set as to reciprocate with phase difference of about 90° with respect to reciprocation of the piston 401 in the same cycle due to pressure change of the working gas in the working space 412 resulting from reciprocation of the piston 401.
A regenerator 404 is arranged outside the first cylinder 403 to enclose the same, and this regenerator 404 separates the expansion space 406 and the compression space 407 from each other. Further, internal heat exchangers 405a and 405b are arranged to enclose the first cylinder 403 through the regenerator 404. The working gas reciprocates between the expansion space 406 and the compression space 407 in response to reciprocation of the displacer 402. The working gas successively permeates the internal heat exchanger 405a, the regenerator 404 and the internal heat exchanger 405b when moving from the expansion space 406 to the compression space 407, and reversely permeates the same when moving backward.
The working gas is treated in the aforementioned manner thereby forming a reverse Stirling heat cycle in the working space 412 and obtaining a low temperature in the expansion space 406. The reverse Stirling heat cycle such as the principle of generation of a low temperature is a known technique, and hence description thereof is omitted.
In the aforementioned conventional Stirling refrigerator, the piston 401 may be hollowed in order to reduce a driving load or the material cost. Further, a gas bearing may be employed for attaining lubrication between the piston 401 and the second cylinder 415. As a structure simultaneously implementing both of these cases, therefore, the section of the piston 401 may conceivably be brought into a structure shown in FIG. 36. A hole connecting the internal space 421 and the compression space 407 with each other is provided on a surface of an outer shell 420 of the piston 401 facing the compression space 407, and a check valve 422 is provided for permitting the working gas passing through this hole to move toward the internal space 421 while inhibiting the same from moving toward the compression space 407. The working gas flowing into the internal space 421 through the check valve 422 effuses out from the piston 401 through a gas bearing hole 423 provided on a surface of the outer shell 420 sliding with the second cylinder 415 since the pressure in the internal space 421 is increased as the piston 401 progresses. Thus, the working gas effusing through the gas bearing hole 423 forms a gas bearing between the piston 401 and the second cylinder 415 for facilitating smooth reciprocation of the piston 401.
In the Stirling refrigerator comprising the aforementioned gas bearing, it follows that the working gas flows into the internal space 421 of the piston 401. In order to reduce the weight, on the other hand, the internal space 421 is desirably increased to the maximum in size. If the internal space 421 of the piston 401 has a large capacity, however, it follows that not only the compression space 407 but also the internal space 421 is compressed when the piston 401 moves toward the compression space 407. If the internal space 421 is wide, the quantity of work in compression is increased. Thus, energy lost as miscellaneous loss is increased.
An object of the first invention is to provide a Stirling engine enabling suppression of the problem described with reference to the first prior art, i.e., reduction of the performance of the Stirling engine or reduction of reliability resulting from clogging in the gas outlet.
The second invention has been proposed in consideration of the circumstances described with reference to the second prior art, and an object thereof is to provide a Stirling engine comprising a tapered surface partially or entirely on either one or both of a contact surface of a porous body with the peripheral wall of an effector and the inner surface of the peripheral wall of the effector and inserting a portion of the porous body having a small outer diameter from a portion of a pressurization chamber in the effector having a large inner diameter so that a load for reducing or enlarging the diameter is applied to the tapered surface, restoring force for enlarging or reducing the diameter is caused on the said tapered surface after insertion of the porous body into the pressurization chamber, and adhesion between the porous body and the peripheral wall of the effector is strong.
Another object of the second invention is to provide a Stirling engine comprising a constraint portion consisting of a viscous synthetic resin material on a contact surface of a porous body with a peripheral wall of an effector for constraining the porous body on the effector through the constraint portion so that the porous body does not move from a prescribed position due to the viscosity of the constraint portion.
Still another object of the second invention is to provide a Stirling engine provided with a constraint portion to enclose the peripheral edge of a through hole on the inner surface of a peripheral wall of an effector for constraining a porous body on the effector through the constraint portion thereby reducing gas flow loss from the outer peripheral portion of the porous body.
A further object of the second invention is to provide a Stirling engine having a porous body provided with a notched portion or a slit to be capable of changing the outer diameter of the porous body by reducing the width of the notched portion or the slit so that the porous body can be readily inserted into a pressurization chamber and restoring force for enlarging the width is caused on the notched portion or the slit after insertion of the porous body into the pressurization chamber thereby attaining strong adhesion between the porous body and the peripheral wall of the effector.
A further object of the second invention is to provide a Stirling engine having a pressurization chamber provided with a step portion and a porous body provided with a projection for stopping the projection by the step portion when inserting the porous body into the pressurization chamber thereby readily arranging the porous body on a prescribed position in the pressurization chamber.
A further object of the second invention is to provide a Stirling engine comprising step portions on two portions in a pressurization chamber through an open end of a through hole for bonding a porous body and the respective ones of the step portions provided on two portions thereby reducing effusion of gas from the outer peripheral portion of the porous body.
A further object of the second invention is to provide a Stirling engine having a porous body so fixed to a peripheral wall of an effector with a pin that the porous body does not move from a prescribed position.
A further object of the second invention is to provide a Stirling engine capable of reinforcing adhesion of a porous body to an effector by preparing the aforementioned porous body from a synthetic resin material while attaining weight reduction of a piston including the porous body and capable of damping vibration and noise in engine operation.
The third invention has been proposed in order to solve the problem described with reference to the third prior art, and an object thereof is to provide a Stirling engine attaining reduction of miscellaneous loss following gas flowage in the Stirling engine and miscellaneous loss following vibration of the Stirling engine itself.